This application makes reference to and claims all benefits accruing under 35 U.S.C. Section 119 from an application entitled, xe2x80x9cHelical Antennaxe2x80x9d, filed in the Japanese Patent Office on Aug. 31, 1999 and there duly assigned Serial No. 11-246433.
1. Field of the Invention
The present invention relates generally to an antenna structure. More particularly, the present invention relates to a helical antenna.
2. Description of the Related Art
A Broadcasting Satellite (BS) antenna, which uses circularly polarized waves, is often used as a satellite telephone terrestrial base station (hereinafter, referred to as a satellite telephone). Such an antenna requires a uniform antenna gain over a wide range of angle in order to acquire necessary radio waves from a plurality of satellites in the air for communication. FIG. 6 illustrates a radiation pattern of an ideal antenna gain. As shown in FIG. 6, it is desirable to have an antenna gain G1 in the vertical direction relative to the ground that is almost equal to an antenna gain G2 at a low elevation angle of about 60xc2x0. If such a uniform antenna gain is possible over a wide angle, antenna 31 can obtain an almost uniform antenna gain regardless of the location of a satellite 32 and in turn, perform high-quality communication via the satellite 32.
In addition to the helical antenna, conical, patch, and conical spiral antennas can be used since the satellite communication is normally performed using the circularly polarized waves. The conical spiral antenna is disclosed in a Japanese patent publication No. 5-251921 and the background information is inferred therefrom. The disclosed conical spiral antenna is made by etching a copper film on a dielectric circular cone to form a spiral coil. By doing so, it is possible to reduce the size of the antenna and increase the operating frequency band of the antenna. However, since the conical, patch and conical spiral antennas are expensive, a helical antenna with a short 4-line type helical antenna is widely used. Lately, an automobile equipped with the 4-line type helical antenna for a satellite telephone or satellite mobile telephone communication has come into wide public use. FIG. 7 illustrates an automobile mounted with the 4-line type helical antenna.
However, it is difficult for the 4-line type helical antenna to have a uniform gain or to increase the antenna gain at a low elevation angle, as compared with the other antennas. Therefore, when the satellite is positioned at a low elevation angle respect to the helical antenna, it is not possible for the satellite telephone attached to the vehicle to maintain or perform high-quality communication. Moreover, when the 4-line type helical antenna is attached to the chassis (or iron board) of the automobile, as shown in FIG. 7, the chassis functions as a ground plate for the antenna. That is, when the radio waves arrive at the antenna, an induced voltage occurs at the antenna in such a way that the re-radiation waves are radiated from the antenna. These re-radiation waves are flown on the chassis as a zero-phase-sequence current of the antenna current. As a result, the radiation pattern of the antenna unit or a vertical axial ratio of the radiation pattern may be distorted, thus making it difficult to obtain a uniform antenna gain over a wide angle and causes communication error.
It is, therefore, an object of the present invention to provide a 4-line type helical antenna with a simple structure to obtain a uniform antenna gain even at a low elevation angle, and exhibit a radiation pattern that is not affected even when the antenna is mounted on the chassis of an automobile.
To achieve the above and other objects, a helical antenna is provided for use in a satellite communication. The helical antenna includes a circular cone surface made of a metal interposed between an antenna body for transmitting and receiving radio waves to/from a satellite, and a satellite terminal for transmitting and receiving the radio waves to/from the antenna body, wherein the circular cone reflects the radio waves of the antenna body. By interposing the circular cone between the antenna body and the satellite terminal, it is possible to efficiently reflect the radio waves of the antenna body on the circular cone, thereby obtaining a uniform radiation pattern over a wide angle.
Preferably, the antenna body has antenna conductors that are spirally formed thereon, and the circular cone is tapered at a predetermined angle to uniformly reflect the radio waves of the antenna body. The circular cone is fixed to one end of the antenna body so that the tapered angle of the circular cone should be uniformly allocated with respect to the axis of the antenna body. By doing so, it is possible to obtain a uniform antenna gain even at a relatively low elevation angle.
Preferably, the tapered angle of the circular cone is determined in such a way that an antenna gain based on a radiation pattern of the antenna body should not become lower than a predetermined value even at an elevation angle of about 30xc2x0 from the horizontal reference line. That is, by selecting an optimal tapered angle of the circular cone, the antenna gain is scarcely attenuated even at the low elevation angle of about 30xc2x0 from the horizontal reference line. In this regard, if the tapered angle is preferably set to 30xc2x0 with respect to the virtual axis of the circular cone, the antenna gain may not be attenuated below 5 dB.
Preferably, the tapered angle of the circular cone is determined in such a way that the radio waves of the antenna body should not be reflected on the ground when the helical antenna is attached to the ground. That is, the radio waves of the antenna body are effectively reflected by the circular cone tapered at a predetermined angle. Therefore, even when the helical antenna is mounted on the chassis of the automobile, the zero-phase-sequence current of the antenna radio waves flows on the chassis will have no effect, thus preventing the radiation pattern from being distorted. In addition, by determining a tapered angle for obtaining a desirable antenna gain, the antenna radio waves are simultaneously reflected to provide a solution for the ground reflection problem.
Preferably, the antenna body and the circular cone arc formed as one structure, and the tapered part of the circular cone is evaporated with a metal.
Preferably, the antenna conductor includes a patterned wired which is formed by etching, printing or firing on an isolation bar. By doing so, the productivity is further increased and the cost is reduced. In addition, by applying the inventive structure to a 4-line type helical antenna, it is possible to more efficiently maintain the antenna gain and provide a solution for the ground reflection problem.